The present invention relates to a method for the thermochemical surface treatments of materials in a reactive gas plasma, as well as to products produced with this method. The plasma heat treatment in reactive gases or gas mixtures is used industrially for such purposes as nitriding and nitrocarburizing. Compared to conventional heat-treatment methods with gas media, salt-bath media or powdery donor media, plasma heat treatment in reactive gases or gas mixtures has the advantage that a plurality of different types of layers can be produced, the properties of which can be adapted to the particular operational requirements (cf. Th. LAMPE, Plasma Treatment of Iron Materials in Nitrogen- and Carbon-Containing Gas Mixtures, Progress Reports of the VDI series 5, No. 93, Dusseldorf 1985, pages 1 ff).
As a rule, direct-current glow-discharge has been used in the past for the industrial application of this plasma heat treatment. Because of the high kinetic energy of the ions produced in the plasma, the treatment was carried out in the area of the so-called anomalous unstable and high-current glow discharge. The important process parameters, available for this procedure, are the discharge conditions of the glow discharge (type of current, current density and voltage), the composition of the reactive gases, the gas pressure, the gas throughput, the temperature of the substrate, the duration of the treatment and the cooling conditions.
In practice, plasma heat-treatment installations predominantly work with a constant direct current, the discharge voltages lying between 300 and 1500 V. Depending on the conditions, the power densities amount to between 0.1 and 4 W/cm.sup.2 (LAMPE, loc. cit. 5. 47). Gas mixtures of nitrogen, hydrocarbon and hydrogen or argon are used for nitrocarburizing and carbonitriding (W. L. GRUBE et al., J. Heat Treating 2 (1982), 211-216) and pure hydrocarbons ((CH4, C3H.sub.8) or mixture of these gases with hydrogen and nitrogen for carburizing (V. N. BLINOV, Metal Sci. Heat Treatm. 24 (1982), 45-47). For plasma nitriding or plasma nitrocarburizing, temperatures between 350.degree. and 700.degree. C. were usually employed 9B. EDENHOFER et al., Harterei-Tech. Mitt. 35 (1980), 175-181) and for carburizing and carbonitriding temperatures between 800.degree. and 1000.degree. C. (W. L. GRUBE et al., J. Heat Treating 2 (1982), 211-216). The gas pressure employed in these processes generally was between 0.5 and 15 mbar, corresponding to 50 to 1500 pascal (Pa), the treatment being carried out under a static partial vacuum (without flow of the working gas) as well as under a dynamic partial vacuum (with flow of the working gas) in an open cycle (cf. LAMPE loc. cit., pages 47/48, H. WILHELMI et al., Harterei-Tech. Mitt. 37 (1982), 263-269, T. WIERZCHON et al., Harterei-Tech. Mitt. 36 (1981), 189-193). This gas pressure is required, on the one hand, to ensure reliable operation of the glow discharge at voltages below 2 kV and, on the other, to improve the relatively modest yields of the process, which may be attributed to the face that the ionized portion of the reaction gas during the glow discharge is less than 1%. It follows from this that almost the whole of the energy is used under these conditions to heat the substrate to the treatment temperature and not to introduce ions into the surface of this substrate. Accordingly, these methods of the state of the art can be regarded as thermochemical methods, in which the heat is supplied over the plasma and the ion process merely represents a subsidiary aspect and a slight improvement of the method.